(1) Field of the Invention:
This invention pertains to a method for the production of reconstituted wood boards. More particularly this invention pertains to a method whereby particleboard or fiberboard can be produced in a press by introducing saturated steam into a partially compressed mat consisting of lignocellulosic material mixed with thermosetting resin binder, causing a sudden rise in interior mat temperature without a corresponding increase in mat moisture content, thereby allowing a reduction in pressing time.
(2) Description of the Prior Art:
In the production of boards from reconstituted wood a thermosetting resin binder is mixed with wood fibers or particles to form a mat. The mat is then placed between two platens and pressed into the composite panel. During pressing, heat is supplied to the mat to plasticize it, thereby making the mat easier to compress, and also to cure the thermosetting resin binder. The time spent in pressing is the major "bottleneck" in the production of particleboard, and to a large extent is dependent on the mechanism of heat transfer used to supply heat to the mat.
In conventional pressing of reconstituted wood heat is transferred to the mat by conduction from heated platen surfaces. This method requires some time to raise the mat's core temperature to a level sufficient to cure the thermosetting resin binder and to complete the panel formation. This is particularly a problem with thick mats because press time does not vary linearly with mat thickness.
Attempts have been made to reduce press time in conduction pressing by increasing the temperature of the platens. However, only slight reductions in press time were achieved, and increased platen temperature also resulted in burning the panels.
In contrast to conduction pressing, reductions in press time have been achieved by transferring heat to mats convectively, thereby taking advantage of the natural porosity of the mats. Convective heating effects cures of mat thicknesses not possible with conduction pressing. A well-known method using convective heat transfer is the "steam shock" or "steam jet" technique wherein mats laden with surface moisture are contacted with hot platens which vaporize the water. The steam thus created moves quickly toward the center of the mat, thereby raising the core temperature. As more water is used, the core temperature will increase. However, more press time is then required to rid the mat of excess moisture, and the surface of the panel often blisters.
Later methods introduce steam directly into the mat to convect heat. One method passes low-pressure steam through the mat from one edge to the other. However, temperature and moisture gradients develop along the direction of steam flow, and these gradients can result in panel warpage. In addition the maximum mat center temperature on the discharge side is 212.degree. F., which will not cure some thermosetting resins.
To reach higher mat temperatures and to prevent moisture formation by steam condensation another method (see Corbin et al U.S. Pat. No. 3,280,237) passes superheated steam from a top, apertured platen through a partially compressed mat. The pressure differential created by high steam injection pressures allows evacuation of the steam to the atmosphere through the edges of the mat. However, using superheated steam for the commercial production of reconstituted wood boards rather than using lower quality steam is expensive due to increased equipment costs for superheaters and increased energy costs to add heat to lower quality steam.
Another method describes a continuous press for producing particleboard wherein superheated steam is injected into a partially compressed mat from gas-permeable conveyor belts and then exhausted to the atmosphere from the edges of the mat. However, as described previously, the use of superheated steam makes the press operation more costly and incurs higher energy losses than would the use of lower quality steam such as saturated steam.
Recently, a method (see Shen U.S. Pat. No. 3,891,738) has been proposed wherein saturated steam under pressure is introduced into a mat which has been compressed to its final desired thickness. The steam is injected through an apertured platen on one side of the mat and exhausted through another apertured platen on the other side of the mat. The mat is confined in a sealed chamber, and an intricate network of passageways and valves restricts the exhaust. By these means high temperatures and pressures are maintained in the chamber. However, this method adds complicated equipment and expense to the press operation in order to insure a tight seal. Furthermore, compressing the mat to its final thickness prior to injecting steam is energy inefficient for two reasons. First, mat porosity is thereby diminished, consequently hindering steam flow between wood flakes and reducing convective heat transfer. Second, high pressure is needed to close the press when the mat has not first been plasticized by steam injection; when a mat has been plasticized first, the required closing pressure is much less, and consequently less energy is consumed.
Press time can also be reduced by catalyzing the reaction between constituents of the thermosetting resin binder and hence speeding its cure. One method adds a gaseous catalyst and resin binder constituents to a fluidized bed of lignocellulosic material. The binder is then formed during subsequent hot pressing of the material. However, this method is limited to catalyst addition prior to mat formation. Therefore, this method cannot be used as a means to add catalyst during pressing of the mat.